Process for preparing purine nucleosides

ABSTRACT

The present invention provides for the preparation β-adenine nucleosides by coupling an adenine derivative containing an unprotected exocyclic amino group at the C-6 position and a blocked arabinofuranosyl derivative, in the presence of a base and solvent. The present invention also provides for the stereoselective preparation of 2-deoxy-β-D-adenine nucleosides wherein a blocked 2-deoxy-α-D-arabinofuranosyl halide is coupled with the salt of an adenine derivative. The forgoing aspects of the present invention are utilized in the preparation of a clofarabine composition wherein the ratio of β to α-anomer is at least 99:1.

[0001] This application claims priority to U.S. provisional applicationNo. 60/309,590, filed Aug. 2, 2001, and hereby incorporated byreference.

FIELD OF THE INVENTION

[0002] The present invention relates generally to the chemicalpreparation of purine nucleosides. More specifically, the inventionrelates to the coupling of an adenine derivative with a blockedarabinofuranosyl to form a β-D-adenine nucleoside. Such nucleosides arevaluable compounds in the field of cancer therapy and as anti-viralagents.

BACKGROUND OF THE INVENTION

[0003] A number of β-D-purine nucleosides derived from adenine areuseful as antitumor and antiviral agents. An important step in thesynthesis of such agents is the formation of the N-glycoside bondbetween the adenine nucleobase and an arabinofuranosyl derivative. Thecoupling reactions used to form the N-glycoside bond of2′-deoxynucleosides have typically resulted in the formation of amixture of α and β-anomers.

[0004] Nucleosides have been synthesized by fusion glycosylation,wherein the reaction is carried out in the absence of solvent at atemperature sufficient to convert the reactants to a molten phase. E.g.,2,6-dichloropurine has been coupled under fusion conditions with5-O-benzyl-2-deoxy-1,3-di-O-acetyl-2-fluroarabinose to form a2′-fluoroarabinonucleoside in 27% yield (Wright et al., J. Org. Chem.34:2632, 1969). Another synthetic method utilizes silylated nucleobasederivatives, e.g., a silylated nucleobase has been coupled with aperacetylated deoxy-sugar in the presence of a solvent and a FriedelCrafts catalyst (Vorbruggen et al., J. Org. Chem. 41:, 2084, 1976). Thismethod has been modified by incorporating a sulfonate leaving group inthe deoxy-sugar in the synthesis of 2′-deoxy-2′-difluoronucleosides(U.S. Pat. No. 4,526,988; U.S. Pat. No. 4,965,374).

[0005] High yields of 2′-deoxy-2′-fluoro-pyrimidine nucleosides wereobtained from refluxing pyrimidines with2-deoxy-2-fluoro-3,5-di-O-benzoyl-α-O-arabinofuranosyl bromide. (Howellet al., J. Org. Chem. 53:85-88, 1988). It was found that use of solventswith lower dielectric constants produced have higher β:α anomer ratios.It was postulated that such solvents favored an S_(N) 2 reaction,whereas solvents with higher dielectric constants favored production ofα-anomers via an ionic S_(N) 1 pathway.

[0006] Anion glycosylation procedures have also been used to prepare2′-deoxy-2′-fluoropurine nucleosides. EP 428109 discloses the couplingof the sodium salt of 6-chloropurine, formed by sodium hydride, with3,5-dibenzyl-α-D-arabinofuranosyl bromide using conditions that favorS_(N) 2 displacement. Use of 1:1 acetonitrile/methylene chlorideresulted in a nucleoside product with a β:α anomer ratio 10:1, asopposed to a ratio of 3.4:1 observed when using a silylated purinereactant. In regard to the use of adenine salts, the amino substituentat the C-6 position was protected as a benzoyl derivative during thecoupling reaction. Protecting the exocyclic amino group precludes theformation of arabinofuranosyl adducts which otherwise may be expected tobe produced (e.g., Ubukata et al., Tetrahedron Lett., 27:3907-3908,1986; Ubukata et al., Agric. Biol. Chem., 52: 1117-1122, 1988; Searle etal., J. Org. Chem., 60:4296-4298, 1995; Baraldi et al., J. Med. Chem.,41:3174-3185, 1998). The preparation of α and β anomers of2′-deoxy-2′-fluoropurine and 2′-difluoropurine nucleosides by anionglycosylation are disclosed by U.S. Pat. No. 5,744,597 and U.S. Pat. No.5,281,357, with β-anomer enriched nucleosides prepared in a β:α anomerratio of greater than 1:1 to about 10:1 and from greater that 1:1 toabout 7:1 respectively. In regard to purines substituted with exocyclicamino groups, both patents again disclose protecting such groups duringcoupling to an appropriate sugar moiety. U.S. Pat. No. 5,281,357 alsodiscloses the effect of solvents on the β:α anomer ratio of9-[1-(2′-deoxy-2′,2′-difluoro-3′,5′-di-O-benzoyl-D-ribofuranosyl)]-2,6-dipivalamidopurineprepared by coupling the potassium salt of 2,6-dipivalamidopurine withan α anomer enriched preparation of2-deoxy-2,2-difluoro-D-ribofuranosyl-3,5-dibenzoyl-1-trifluoromethanesulfonate. There was no correlation between thedielectric constant of the six solvents used and the β:α anomer ratio,e.g. ethyl acetate and acetonitrile both gave the same ratio of 1.6:1.t-Butyl alcohol gave the highest β:α anomer ratio of 3.5:1.

[0007] Despite the preparative methods for purine nucleosides known inthe art, there is still a need for economically preferable, effectiveand efficient process for the preparation of these compounds. The objectof the present invention is to provide such a process. Further objectsare to minimize the number of process reaction steps and to provide aprocess that is readily scalable for the production of commercial-scalequantities. Other objects and advantages will become apparent to personsskilled in the art and familiar with the background references from acareful reading of this specification.

SUMMARY OF THE INVENTION

[0008] In its most general terms, one aspect of the present inventionprovides for the preparation of β-adenine nucleosides by coupling anadenine derivative containing an unprotected exocyclic amino group atthe C-6 position, and a blocked arabinofuranosyl derivative. Inpreferred embodiments, this reaction can be depicted as:

[0009] R¹ is hydrogen, halogen or —OR⁶, wherein R⁶ is a hydroxyprotecting group. In a preferred embodiment R¹ is fluoro. R² and R³ arehydroxy-protecting groups. In preferred embodiments R², R³ and R⁶ areindependently benzoyl or acetyl. R⁴ is a leaving group. Suitable leavinggroups include, halo, fluorosulfonyl, alkylsulfonyloxy,trifluoroalkylsulfonyloxy and arylsulfonyloxy. In a preferredembodiment, R⁴ is bromo. R⁵ is hydrogen, halogen or —NH₂. In preferredembodiments, R⁵ is chloro or fluoro.

[0010] Surprisingly, this reaction proceeds without substantialproduction of adducts resulting from addition of the blockedarabinofuranosyl (1) with the exocyclic amino group at the C-6 positionof compound (2) (hereinafter termed “C-6 exocyclic amino group”), whichremains unprotected during the reaction, and/or the nitrogen at the N-7position of the adenine ring. An example of an undesired C-6 exocyclicamino group by-product adduct is represented by the following formula:

[0011] For the purposes of the present invention, and in light of theobjective to provide an economically preferable, effective and efficientprocess, “substantial formation” means conversion of about 40% of theadenine derivative of formula (2) to a by-product adduct or adductsresulting from addition of the blocked arabinofuranosyl of formula (1)to the unprotected C-6 exocyclic amino group and/or N-7 position ofcompound (2). In embodiments wherein R⁵ is —NH₂ (hereinafter termed “R⁵—NH₂ group”), “substantial formation” means conversion of about 40% ofthe adenine derivative of formula (2) to by-product adduct(s) resultingfrom addition of the blocked arabinofuranosyl of formula (1) to theunprotected C-6 exocyclic amino group and/or N-7 position and/or the R⁵—NH₂ group of compound (2).

[0012] Even more surprising is that the reaction can proceed withouteven a significant production of adducts resulting from addition of theblocked arabinofuranosyl (1) with the C-6 exocyclic amino group and/orN-7 position of compound (2). For the purposes of the present invention,“significant production” means conversion of about 5% of the adeninederivative of formula (2) to a by-product adduct or adducts resultingfrom addition of the blocked arabinofuranosyl (1) to the unprotected C-6exocyclic amino group and/or N-7 position of compound (2). Inembodiments wherein R⁵ is —NH_(2,) “significant production” meansconversion of about 5% of the adenine derivative of formula (2) to aby-product adduct(s) resulting from addition of the blockedarabinofuranosyl of formula (1) to the unprotected C-6 exocyclic aminogroup and/or N-7 position and/or the R⁵ —NH2 group of compound (2).

[0013] Useful bases are generally those with a pKa in water of 15 orgreater. In preferred embodiments, the base is an alkali metal base,more preferred being a potassium base. In preferred embodiments, thebase is a sterically hindered base, e.g., potassium t-butoxide orpotassium t-amylate. Suitable inert solvents include, but are notlimited to, t-butyl alcohol, acetonitrile, dichloromethane,dichloroethane, t-amyl alcohol, tetrahydrofuran or mixtures thereof. Inpreferred embodiments, the solvent or solvent mixture has a boilingpoint of about 80° C. or greater.

[0014] The process of the present invention also further comprisesde-protection of the blocked carbohydrate moiety to form a β-nucleosideof the formula:

[0015] wherein, R¹ and R⁵ are as defined above.

[0016] In some embodiments, the adenine derivative is 2-chloroadenineand the blocked arabinofuranosyl derivative is a2-deoxy-2-fluoro-arabinofuranosyl derivative, whereupon the resultingβ-nucleoside is a 2-chloro-9-(2′-deoxy-2′-fluoro-β-D-arabinofuranosyl)adenine derivative. The reaction can be depicted as:

[0017] wherein R ², R³ and R⁴ are as defined above. The process alsofurther comprises de-protecting the carbohydrate moiety to form2-chloro-9-(2′-deoxy-2′-fluoro-β-D-arabinofuranosyl) adenine, also knownas clofarabine.

[0018] Another aspect of the invention is the discovery of thesurprising steroselectivity that can be achieved in the production2′-deoxy-2′-halo-β-D-adenine nucleosides wherein such nucleosides arealso produced in high yield. This reaction can be depicted as:

[0019] R⁷ and R⁸ are independently halogen, M⁺is potassium, and R², R³,and R⁵ are as defined above. Halogen includes bromo, fluoro, chloro andiodo. In a preferred embodiment R⁸ is fluoro. In various embodiments R⁷is chloro or, preferably, bromo. In some embodiments, the processfurther comprises the addition of calcium hydride. Suitable inertsolvents include t-butyl alcohol, acetonitrile, dichloromethane,dichloroethane, t-amyl alcohol, tetrahydrofuran or mixtures thereof. Inpreferred embodiments, the solvent is a mixture of t-butyl alcohol andacetonitrile, or a mixture of t-butyl alcohol and dichloroethane, or amixture of dichloroethane and acetonitrile, or a mixture of t-amylalcohol and dichloroethane, or a mixture of t-amyl alcohol andacetonitrile, or a mixture of t-amyl alcohol, acetonitrile anddichloromethane, or a mixture of t-amyl alcohol, acetonitrile anddichloroethane. In preferred embodiments, the solvent or solvent mixturehas a boiling point of about 80° C. or greater.

[0020] In some embodiments, the adenine derivative salt (10) is formedin situ by the reaction of a potassium base with the correspondingadenine derivative (2). In preferred embodiments, the base is potassiumt-butoxide or potassium t-amylate.

[0021] In various embodiments of the invention, the coupling reactionproduces a preparation wherein the ratio of the β-anomer of formula (11)to the α-anomer of formula (12) is at least about 10:1, or preferably isat least about 15:1, or more preferable is at least about 20:1. Thus,the anomer ratio may be 10:1 or greater, 15:1 or greater or 20:1 orgreater. In preferred embodiments the β-anomer of formula (11) isprepared in a yield of about 40% or greater. In more preferredembodiments, the β-anomer of formula (11) is prepared in yields of about50% or greater or about 80% or greater.

[0022] The process of the present invention may also further comprisesisolation of the β-anomer (11) by subjecting the mixture of β andα-anomers to recrystallization or by a re-slurry procedure. In apreferred embodiment, the further purification comprises reslurry frommethanol or crystallization from a mixture of butyl acetate and heptane.In various embodiments, the purified preparation comprises a mixture ofnucleosides wherein the ratio of the β-anomer of formula (11) to theα-anomer of formula (12) is at least about 20:1, or least about 40: 1,or at least about 60:1.

[0023] The process also further comprises de-protection of the blockedcarbohydrate moiety of the protected β-anomer to form a β-nucleoside ofthe formula:

[0024] wherein, R⁵ and R⁸ are as defined above. When R⁵ is chloro and R⁸is fluoro, the unblocked β-nucleoside of formula (13) is2-chloro-9-(2′-deoxy-2′-fluoro-β-D-arabinofuranosyl) adenine.

[0025] Another aspect of the present invention is a multi-step processfor the preparation of a composition comprising2-chloro-9-(2′-deoxy-2′-fluoro-β-D-arabinofuranosyl) adenine. Thiscomprises the integration of the other aspects of the present inventioninto an economically preferable, effective and efficient synthesis andisolation of 2-chloro-9-(2′-deoxy-2′-fluoro-β-D-arabinofuranosyl)adenine. This process minimizes the number of steps in part by notrequiring protection of the C-6 exocyclic amino group. In addition, thesurprising stereoselective preference for the β-anomer in part enablesthe preparation of a composition with an β:α anomer ratio of at least99:1 or in preferred embodiments is about 400:1 or greater, about 500:1or greater or about 1000:1 or greater, without utilizing a preparativechromatography step for the purification of the β-anomer. The absence ofa chromatographic step is a major advantage in regard to an economicallypreferable commercial-scale process.

[0026] The process comprises reacting3,5-O-dibenzoyl-2-deoxy-2-fluoro-α-D-arabinofuranosyl bromide with a2-chloroadenine potassium salt of the formula:

[0027] in the presence of a solvent to form2-chloro-9-(3′,5′-O-dibenzoyl-2′-deoxy-2′-fluoro-β-D-arabinofuranosyl)adenine. The C-6 exocyclic amino group of the 2-chloroadenine potassiumsalt is not protected during the process. The2-chloro-9-(3′,5′-O-dibenzoyl-2′-deoxy-2′-fluoro-β-D-arabinofuranosyl)adenine is then de-protected to form2-chloro-9-(2′-deoxy-2′-fluoro-β-D-arabinofuranosyl) adenine, which isthen isolated to provide a composition comprising2-chloro-9-(2′-deoxy-2′-fluoro-β-D-arabinofuranosyl) adenine. In someembodiments, wherein the composition produced by the multi-step process,as described above, also comprises2-chloro-9-(2′-deoxy-2′-fluoro-β-D-arabinofuranosyl) adenine, the2-chloro-9-(2′-deoxy-2′-fluoro-β-D-arabinofuranosyl) adenine issubstantially pure. For the purposes of the present invention,substantially pure 2-chloro-9-(2′-deoxy-2′-fluoro-β-D-arabinofuranosyl)adenine means that the ratio of β-anomer to α-anomer as measured by highpressure liquid chromatography and spectrophotometric analysis, is atleast 99:1.

[0028] The process may further comprise isolating the2-chloro-9-(3′,5′-O-dibenzoyl-2′-deoxy-2′-fluoro-β-D-arabinofuranosyl)adenine before the deprotection step. In some embodiments, thisisolation may comprise reslurry and/or recrystallization, which may beeffected by use of methanol or by use of a mixture of butyl acetate andheptane. In other embodiments, the isolation of2-chloro-9-(2′-deoxy-2′-fluoro-β-D-arabinofuranosyl) adenine alsocomprises recrystallization. In some embodiments, the recrystallizationis from methanol.

[0029] In some embodiments, the 2-chloroadenine potassium salt isprepared in situ by the reaction of a potassium base with2-chloroadenine in a suitable inert solvent. In preferred embodiments,the base is potassium t-butoxide or potassium t-amylate. Suitable inertsolvents include t-butyl alcohol, acetonitrile, dichloromethane,dichloroethane, t-amyl alcohol, tetrahydrofuran or mixtures thereof. Inpreferred embodiments, the solvent is a mixture of t-butyl alcohol andacetonitrile, or a mixture of t-butyl alcohol and dichloroethane, or amixture of dichloroethane and acetonitrile, or a mixture of t-amylalcohol and dichloroethane, or a mixture of t-amyl alcohol andacetonitrile, or a mixture of t-amyl alcohol, acetonitrile anddichloromethane, or a mixture of t-amyl alcohol, acetonitrile anddichloroethane.

BRIEF DESCRIPTION OF THE DRAWINGS

[0030] The following drawings form part of the present specification andare included to further demonstrate certain aspects of the presentinvention. The invention may be better understood by reference to one ormore of these drawings in combination with the detailed description ofspecific embodiments presented herein.

[0031]FIG. 1 Schematic representing potential rationale for the effectof potassium in the stereoselective production of2′-deoxy-2′-fluoro-β-D-adenine nucleosides. R², R³ and R⁵ are as definedabove.

[0032]FIG. 2 Schematic of expected conformations of the relevant protonsand fluorine atoms for2-chloro-9-(2′-deoxy-2′-halo-β-D-arabinofuranosyl) adenine (clofarabine)(21) and 2-chloro-9-(2′-deoxy-2′-fluoro-α-D-arabinofuranosyl) adenine(epi-clofarabine) (22).

[0033]FIG. 3 Partial 1H NMR for2-chloro-9-(2′-deoxy-2′-fluoro-β-D-arabinofuranosyl) adenine(clofarabine) (21).

[0034]FIG. 4 Partial 1H NMR for2-chloro-9-(2′-deoxy-2′-fluoro-(α-D-arabinofuranosyl) adenine(epi-clofarabine) (22).

DETAILED DESCRIPTION OF THE INVENTION

[0035] 1. Coupling Reactions Utilizing Purine Bases with UnprotectedExocyclic Amino Groups

[0036] One aspect of the present invention provides for the preparationβ-adenine nucleosides by coupling an adenine derivative with anunprotected C-6 exocyclic amino group and a blocked arabinofuranosylderivative, in the presence of a base and solvent. The blockedarabinofuranosyl derivative may be depicted by the structure:

[0037] R¹ is hydrogen, halogen or —OR⁶, wherein R⁶ is a hydroxyprotecting group. Halogens include bromo, chloro, fluoro and iodo. R²and R³ are hydroxy protecting groups. Hydroxy protecting groups areknown in the art as chemical functional groups that can be selectivelyappended to and removed from a hydroxy functionality present in achemical compound to render such functionality inert to chemicalreaction conditions to which the compound is exposed. Hydroxy protectinggroups are described in Greene and Wuts, Protective Groups in OrganicSynthesis, 2d edition, John Wiley & Sons, New York, 1991, and includeformyl, acetyl, propionyl, arylacyl (e.g., benzoyl or substitutedbenzoyl), trityl or monomethoxytrityl, benzyl or substituted benzyl,carbonate derivatives (e.g., phenoxycarbonyl, ethoxycarbonyl andt-butoxycarbonyl), and trisubstituted silyl, including trialkylsilyl(e.g. dimethyl-t-butylsilyl) or diphenylmethylsilyl. In preferredembodiments, the protecting groups are independently benzoyl or acetyl.

[0038] R⁴ is a leaving group, suitable examples of which includehalogen, alkylsulfonyloxy, and arylsulfonyloxy. Halogens include chloro,fluoro, iodo and, in a preferred embodiment, bromo. Blockedα-arabinofuranosyl halides can be prepared by various methods known inthe art employing standard procedures commonly used by one of skill inthe art, e.g., 3,5-O-dibenzoyl-2-deoxy-2-fluoro-α-D-arabinofuranosylbromide (exemplified in Example 1; Tann et al., J. Org. Chem., 50:3644,1985, herein incorporated by reference);3-O-acetyl-5-O-benzyl-2-deoxy-2-fluoro-(α-D-arabinofuranosyl bromide(Fox et al., Carbohydrate Res., 42:233, 1975, herein incorporated byreference); 2,3,5-O-tribenzyl-α-D-arabinofuranosyl chloride (U.S. Pat.No. 5,110,919, herein incorporated by reference); and3,5-O-di-p-toluoyl-2-deoxy-α-arabinofuranosyl chloride (Bhattacharya etal., J. Org. Chem., 28:428 1963; Nuhn et al., Pharmazie, 24:237, 1969,both herein incorporated by reference). Preparation of blockedα-arabinofuranosyl derivatives substituted at the C-1 position withalkylsulfonates and arylsulfonates are disclosed in U.S. Pat. No.5,401,861 and U.S. Pat. No. 5,744,579, both herein incorporated byreference. Alkyl sulfonates include methanesulfonate, ethylsulfonate andbutylsulfonate and substituted alkyl sulfonates include compounds suchas trifluoromethane sulfonate and 1,1,1-trifluoromethanesulfonate.Arylsulfonates includes substituted arylsulfonates such asp-nitrobenzenesulfonate, p-bromobenzenesulfonate,p-methylbenzesulfonate, and the like.

[0039] Useful bases generally have a pKa in water of 15 or greater andare suitable for the formation of a salt of the adenine derivative (2),as depicted by the formula:

[0040] R⁵ is as defined previously and R+is a monovalent cation. Thebase may be an alkali metal base, and in preferred embodiments thealkali metal base is a potassium base. In preferred embodiments, thebase is a sterically hindered base, e.g., potassium t-butoxide orpotassium t-amylate.

[0041] Solvents useful in the present invention are those that are inertin respect to the reaction. Suitable inert solvent include, but are notlimited to, t-butyl alcohol, acetonitrile, dichloromethane,dichloroethane, t-amyl alcohol, tetrahydrofuran or mixtures thereof.

[0042] In a preferred embodiment, the reaction is carried out at roomtemperature. However, in other embodiments the reaction is carried outat elevated or lower temperatures. E.g., the reaction can be carried outat about 40° C., or about 50° C., or about 60° C., or under refluxconditions. Alternatively the reaction can be carried out from about−25° C. to about 25° C., e.g., at about −20° C. or at about −10° C., orat about 0° C., or at about 10° C.

[0043] Wherein an amino group is described as “unprotected,” this meansthat the amino group has not been blocked by an amino protecting group.The use and types of amino protecting functionalities are well known inthe art. Examples are described in Greene and Wuts, Protective Groups inOrganic Synthesis, 2d edition, John Wiley & Sons, New York, 1991.

[0044] The molar ratio of reactants is not considered to be critical andin preferred embodiments approximately equal molar equivalents ofblocked arabinofuranosyl derivative (1), adenine derivative (2) and baseare used. In some embodiments, a slight molar excess (e.g., 1.05 to 1.15equivalents) of adenine derivative (2) and/or base are used. Thepreferred order and manner of addition for any specific embodiment canbe determined by routine experimentation with a view towards bothreaction performance and chemical engineering and productionsconsiderations.

[0045] 2. Stereoselective Preparation of 2-Deoxy-Purine Nucleosides

[0046] Another aspect of the invention is the stereoselectivepreparation of 2-deoxy-β-D-adenine nucleosides. In this process, ablocked 2-deoxy-α-D-arabinofuranosyl halide is coupled with the salt ofan adenine derivative depicted by the formula:

[0047] R⁵ and M⁺are as previously described. Surprisingly, the identityof the cation has a profound effect on the stereoselectivity of thecoupling reaction. Potassium salts produced larger β:α anomer ratiosthan lithium or sodium salts. The salt depicted by formula (10) can beproduced in situ by use of potassium bases and adenine derivatives offormula (2). Suitable bases generally have a pKa in water of 15 orgreater and include potassium t-alkoxide bases, potassium hydroxide andhindered bases include potassium diisopropylamide, potassiumbis(trimethylsilyl)amide, potassium hexamethyldisilazide, potassiumhydride and the like. In preferred embodiments, the base is a stericallyhindered base, e.g., potassium t-butoxide or potassium t-amylate.

[0048] While not being bound by any theory, the preferentialstereoselectivity observed with potassium may be due, e.g., when R⁸ isfluoro and R⁷ is bromo, to an electrostatic attraction between theelectronegative fluorine atom and the hard potassium cation, leading toa preferential β-face attack, as depicted in FIG. 1. The lack ofselectivity of lithium and sodium may be due to a more covalentassociation of the cation with the purine base. The present inventionalso encompasses other cations, such as cesium, that can replacepotassium as a hard cation.

[0049] The solvent employed also has a marked effect on the β:α anomerratio. Generally solvents with a lower dielectric constant favorproduction of the β anomer. But solvent choice is not dictated simply bydielectric constant, in that there is a tendency for an inverserelationship between increasing the β:α anomer ratio and the yield ofthe β and α anomers. This effect presumably relates to the solubility ofreactants and/or intermediates. Suitable solvents include t-butylalcohol, acetonitrile, dichloromethane, dichloroethane, t-amyl alcohol,isoamyl alcohol, tetrahydrofuran or mixtures thereof. In preferredembodiments, the solvent is a mixture of t-butyl alcohol andacetonitrile, or a mixture of t-butyl alcohol and dichloroethane, or amixture of dichloroethane and acetonitrile, or a mixture of t-amylalcohol and dichloroethane, or a mixture of t-amyl alcohol andacetonitrile, or a mixture of t-amyl alcohol, acetonitrile anddichloromethane, or a mixture of t-amyl alcohol, acetonitrile anddichloroethane. Two component mixtures the two solvents may be combinedin the range of about 1:4 to about 1:1 v/v. In three component mixtures,the three solvents may be combined in ratios of about 2:2:1, or about2:1:1, or about 1:1:1.

[0050] In a preferred embodiment, the reaction is carried out at roomtemperature. In other embodiments, elevated or lower temperatures areused. Lowering the temperature of the reaction, such as in the range offrom room temperature to about −25° C., may lead to an increase the β:αanomer ratio. Elevated temperatures can be used in the range from roomtemperature to reflux conditions.

[0051] In some embodiments, calcium hydride is added. The addition ofcalcium hydride generally increases the β:α anomer ratio. This effectmay be due in part to the removal of traces of water from the solvent.

[0052] The molar ratio of reactants is not considered to be critical andin preferred embodiments when the adenine derivative salt (10) isproduced in situ, approximately equal molar equivalents of blockedarabinofuranosyl derivative (9), adenine derivative (2), base and, whenadded, calcium hydride are used. In some embodiments, a slight molarexcess (e.g., 1.05 to 1.15 equivalents) of adenine derivative (2) and/orbase are used. The preferred order and manner of addition for anyspecific embodiment can be determined by routine experimentation with aview towards both reaction performance and chemical engineering andproductions considerations.

EXAMPLES OF THE INVENTION

[0053] Without further elaboration, it is believed that one skilled inthe art can, using the preceding description, utilize the presentinvention to its fullest extent. The following specific examples areintended merely to illustrate the invention and not to limit the scopeof the disclosure or the scope of the claims in any way whatsoever.

EXAMPLE 1

[0054] Preparation of3,5-O-Dibenzoyl-2-deoxy-2-fluoro-α-D-arabinofuranosyl Bromide

[0055] A 1-neck roundbottom flask (100 mL) was equipped with a stir barand nitrogen inlet adapter. The flask was charged with dichloromethane(10.4 mL) and 1, 3, 5-O-tribenzoyl 2-deoxy-2-fluoro-β-D-arabinofuranosyl(16) (2.6 gm, Sigma, St. Louis, Mo.) at room temperature. The solutionwas placed under nitrogen. A 33% solution of hydrogen bromide in aceticacid (0.96 gm) was charged and the resultant mixture stirred for 18 hr.The solvent was removed by rotary evaporation to give an orange residue.This was dissolved in dichloromethane (30 mL) and quenched with sodiumbicarbonate brine (30 mL), whereupon the pH was 7-8. The organic phasewas partitioned and washed with sodium chloride brine (30 mL). Theorganic phase was dried over MgSO₄ and filtered. Solvent removal byrotary evaporation and high vacuum afforded3,5-O-dibenzoyl-2-deoxy-2-fluoro-α-D-arabinofuranosyl bromide (17) as aviscous yellow gum.

EXAMPLE 2

[0056] Preparation of2-Chloro-9-(3,5-O-dibenzoyl-2-deoxy-2-fluoro-β-D-arabinofuranosyl)adenine

[0057]2-Chloro-9-(3,5-O-dibenzoyl-2-deoxy-2-fluoro-β-D-arabinofuranosyl)adenine (19) (Borregaard) was prepared utilizing different bases andnumerous solvent systems and the optional addition of calcium hydride.In the following exemplifications, three preparations are described indetail and other preparations are summarized in Table 1.

[0058] A. Preparation I

[0059] A three neck roundbottom flask was equipped with a temperaturecontroller, nitrogen inlet and outlet tubes, septa and a magnetic stirbar. Chloroadenine (18) (0.45g) was charged as a solid under nitrogen,followed by potassium t-butoxide (0.34 g), acetonitrile (2.3 mL) andt-butyl alcohol (6.9 mL). After stirring for 1 hour at 24° C.-26° C.,3,5-O-dibenzoyl-2-deoxy-2-fluoro-α-D-arabinofuranosyl bromide (17) (1.21gm) was added. The resulting orange suspension was stirred at 24° C.-26°C. for 16 hours. HPLC analysis of an in-process control sample showed a96.6% conversion and a 10.7:1 ratio of β-anomers (19) to α-anomer (20).HPLC analysis utilized a reverse phase system with a Zorbax-SB-C18column and a mobile phase of 80:20 acetonitrile/water with 15% v/vtrifluoroacetic acid at a flow rate of 1 mL/min. at 30° C. Detection wasby spectrophotometric analysis at 263 nm. Conversion is expressed asarea under the curve (a.u.c.) values of (19)+(20)/(18)+(19)+(20)×100.The solvent was evaporated to afford 1.79 g of an orange residue. Tothis was added ethyl acetate (34 mL) and the mixture stirred at ambienttemperature for 1.25 hr and then filtered through filter paper and thepaper rinsed twice with 5 mL of ethyl acetate. Evaporation of thefiltrate solution afforded 1.28 g of light orange crystals (86.8% byHPLC area of the combined anomers). This material still contained asmall amount of 2-chloroadenine (13) by BPLC. The anomeric ratio was11.8:1. The crystals were dissolved with 33 mL ethyl acetate at ambienttemperature to afford a slightly opaque solution. This was filteredthrough filtered through a Celite pad and the filtrate evaporated toafford 1.16 g of crystals. This material still contained a small amountof (13). The problem was remedied by more efficient filtration. Thecrystals were dissolved in 25 mL ethyl acetate overnight at ambienttemperature to give a slightly cloudy solution. This was filteredthrough a Whatman 0.45 mM nylon syringe filter and evaporated to afford1.13 g. This material contained no (18) by HPLC analysis and had ananomeric ratio of 11.9:1 and a yield of 83% with a purity of 98.1%(a.u.c.). Considering the production of anomers (19) and (20), there wasno substantial formation of a by-product adduct formed by reaction of3,5-O-dibenzoyl-2-deoxy-2-fluoro-α-D-arabinofuranosyl bromide (17) withthe unprotected exocyclic amino group of 2-chloroadenine (18). Inaddition, HPLC analysis revealed no substantial formation ofby-products.

[0060] B. Preparation II

[0061] A 3-neck roundbottom flask was equipped with a magnetic stir bar,temperature controller, and nitrogen inlet line and charged with2-chloradenine (18) (0.29 g), followed by acetonitrile (1.6 mL), t-amylalcohol (3.3 mL), potassium tert-butoxide (0.2 g) and calcium hydride(0.069 g). This mixture was stirred at 25° C. for 30 minutes before3,5-O-dibenzoyl-2-deoxy-2-fluoro-α-D-arabinofuranosyl bromide (17) (0.68g gm) dissolved in dichloromethane (3.25 mL) was charged. The orangesolution was stirred for two days whereupon HPLC analysis showed a β:αanomeric ratio of 18.8:1 and a conversion of approximately 67%. Heatingat 40° C. for approximately 4.5 hr resulted in a β:α anomer ratio of18.7:1 and a decrease in the apparent conversion to 63%. The reactionmixture was vacuum filtered and the filter cake washed withdichloromethane (2×12 mL). The filtrate was passed through a nylonsyringe filter and then concentrated by rotary evaporation and highvacuum pumping to afford 0.72 g of material with a β:α anomeric ratio of19:1 and was 88% pure by HPLC (a.u.c.), giving a yield of the anomers(19) and (20) of 77%. In that there was an approximately 77% conversionof the chloroadenine, there was neither substantial nor significantformation of a by-product adduct formed by reaction of3,5-O-dibenzoyl-2-deoxy-2-fluoro-α-D-arabinofuranosyl bromide (17) withthe unprotected exocyclic amino group of 2-chloroadenine (18). Inaddition, HPLC analysis revealed no substantial or significant formationof by-products.

[0062] C. Preparation III

[0063] A 3-neck 100 ml round-bottomed flask equipped with magnetic stirbar, temperature controller, and nitrogen inlet line and charged with2:1 t-amyl alcohol:acetonitrile (9 mL) followed by 2-chloradenine (18)(0.63 g), potassium t-amylate (0.47 g) and calcium hydride (0.15 g).This mixture was stirred at room temperature for 30 minutes before theaddition of 3,5-O-dibenzoyl-2-deoxy-2-fluoro-α-D-arabinofuranosylbromide (17) (1.5 gm) dissolved in 2:1 t-amyl alcohol:acetonitrile (7mL). The solution was stirred for 17 hr. whereupon analysis by HPLCshowed the conversion to be approximately 79% and a β:α anomer ratio of14.5:1. The reaction mixture was vacuum filtered and the residue washedwith 2×5 mL acetonitrile. The filtrate was re-filtered through a 0.45 μnylon filter and then concentrated. The concentrate residue wasdissolved in butyl acetate (5 mL). Heptane (35 mL) was added and theresulting crystals were collected by vacuum filtration and subjected toa high vacuum. HPLC analysis of the crystals indicated a β:α anomerratio of 19.4:1 and a 63% yield of material with a 90% purity (a.u.c.).In that there was an approximately 79% conversion of the chloroadenine,there was no substantial formation of a by-product adduct formed byreaction of 3,5-O-dibenzoyl-2-deoxy-2-fluoro-α-D-arabinofuranosylbromide (17) with the unprotected exocyclic amino group of2-chloroadenine (18). In addition, HPLC analysis revealed no substantialformation of by-products.

[0064] D. Summary of Preparative Methods

[0065] Results of preparative examples in addition to those exemplifiedabove in Preparations I, II and III, are summarized in Table 1.Preparative methods typically used approximately molar equivalents of(17) and (18) and calcium hydride and a slight molar excess of base.TABLE 1 Time B:α Ratio Conversion Isolated Solvent  Base* CaH₂ (hrs)(19)/(20) %† Yield (%) 2:1 KOtBu + 14 17 54 ND†† tBuOH/DCE 1:2 KOtBu +14 20.1 60 ND DCE/tAmOH 1:4 KOtBu + 14 20.5 58 ND DCE/tAmOH 1:2:2KOtBu + 14 22.1 74 ND MeCN/DCE/ tAmOH 52% tBuOH KOtBu + 26 10.7 90 4248% MeCN 52% tBuOH KOtBu − 22 10.7 84 77 48% MeCN 52% tBuOH KOtBu − 2111 86 79 48% MeCN 51% amyl KOtBu − 17 13.1 80 83 alcohol 49% MeCN 2:2:1KOtBu + 85 18.7 71 80 CH₂Cl₂: tAmOH: MeCN 2:1 KOtBu + 85 12.7 70 84tAmOH: MeCN 2:1 KOtBu + 85 13.1 77 89 tAmOH: MeCN 2:2:1 CH₂Cl₂: KOtBu +69 13.9 73 41 tAmOH: MeCN 2:1 tAmOH: K − 18 19.6 76 80 MeCN t-amylate1:1 t-AmOH: K − 18 13.3 79 84 MeCN t-amylate 1:2 tAmOH: K − 18 6.73 92ND MeCN t-amylate 2:1:1 tAmOH: KOtBu + 16 20.3 79 48 MeCN:CH₂Cl₂

EXAMPLE 3

[0066] Purification of2-Chloro-9-(3,5-O-dibenzoyl-2-deoxy-2-fluoro-β-D-arabinofuranosyl)adenineby Re-slurry

[0067] A re-slurry step utilizing methanol reflux was used to purifycompound (19). I necessary, the pH should be adjusted to 6.0 prior tothis step to prevent deprotection during the re-slurry step. Given thatthe re-slurry must involve an equilibrium between the solid and solutionphases, a period of time is required for this equilibrium to becomeestablished under a given set of experimental conditions. Thus, thetimes required for equilibration by monitoring the anomeric compositionof slurries at different solvent ratios and temperatures were examined.Three salient features became apparent: (1) a hot re-slurry resulted ingreater amounts of (19) in the solution at equilibrium; (2) the amountof (19) in solution phase increases over time as equilibrium isapproached for the hot re-slurry and decreases over time for a roomtemperature re-slurry; and (3), equilibrium is essentially achieved at 5hours under hot or room-temperature re-slurry conditions, although aslight change is observed under room temperature conditions overovernight stirring. The room temperature re-slurry produced a greateranomeric increase. It was concluded that a re-slurry at roomtemperature, for at least 5 hours, followed by a 1 hour cooling andfiltration results in the best recovery and anomeric ration. Results ofthis method are shown in Table 2 for 20 gm runs undertaken in a 1Lreactor. TABLE 2 Initial ratio Final ratio Conditions† (19)/(20)‡(19)/(20) Mass recovery* A 19 79 62 B 20 39 69 B 24 66 74

EXAMPLE 4

[0068] Hydrolysis of Condensation Product to Afford2-Chloro-9-(3′,5′-O-dibenzoyl-2′-deoxy-2′-fluoro-β-D-arabinofuranosyl)adenine

[0069] Because methyl benzoate is a liquid and is readily soluble inmany organic solvents, cleavage of benzyl groups with sodium methoxidewas preferred. A 250 ml, multi-neck flask, equipped with a thermocouple,magnetic stirrer, nitrogen purge and reflux condenser, was charged with(19) (8.42 gm, 16.45 mmol) and 15 ml methanol at ambient temperature.Stirring was started ands the mixture heated to 38° C. The reaction wascharged with sodium methoxide (62 μl, 0.329 mmol). The reaction mixturewas stirred at 38° C. for 7 hours, heating was them shut off and themixture cooled to ambient temperature and stirred overnight. The pH wasadjusted to 5.0 with acetic acid. The reaction flask was cooled in anice bath 2 hours and the reaction mixture was filtered and the flask andfiltercake were washed with 9.5 ml methanol. The wet solid and 105 mlmethanol were charged to a 250 ml, multi-neck flask, equipped with athermocouple, magnetic stirrer, nitrogen purge and reflux condenser,stirred and heated to reflux. The hot solution was filtered and filtratetransferred to the original reaction flask, wherein the mixture wascooled to ambient temperature. The mixture was cooled in and ice/waterbath for 0.5 hour and the mixture filtered and flask and filtercakerinsed with 9.8 ml methanol. The wet solid was dried in a vacuum oven toproduced (21) at a yield of 69.4% with a purity of 99.14 (a.u.c.). Noα-amoner was detectable by HPLC.

[0070] Further examples of the deprotection method with varyingconditions are shown in Table 3. TABLE 3 mmol NaOMe MeOH Temp. % MassAnomeric HPLC (19) eq. mL/g ° C. Recovery Ratio Area (%) 1.68 0.015 4 2568.8 338/1 98.0 2.05 0.100 20 25 58.9 415/1 99.6 2.11 0.010 20 25 58.4469/1 93.7 2.09 0.010 4 25 64.2 248/1 99.0 2.03 0.100 4 25 68.2 126/198.6 2.82 0.055 12 38 60.2 330/1 99.1 3.07 0.055 12 38 66.9 521/1 99.02.94 0.055 12 38 64.9  ∞/1 99.6 2.01 0.100 4 50 62.6 1657/1  99.4 2.070.010 4 50 64.3 521/1 98.9 1.97 0.010 20 50 59.3 432/1 99.4 1.99 0.10020 50 61.0 397/1 61.0 17.05  0.020 8 25 53.5 988/1 98.8

EXAMPLE 5

[0071] NMR Designations for Clofarabine and Epi-Clofarabine

[0072] Pooled preparations of anomeric mixtures of (19) and (20) werepooled and de-protected by removal of the benzoyl groups by treatmentwith sodium methoxide and methanol. The resulting clofarabine andepi-clofarabine were isolated by preparative HPLC. In a typical run, 60mg of crude sample was dissolved in 1.4 mL of the mobile phase, i.e. 1:9(v/v) acetonitrile/water, for injection onto a Phenomenex Progidy C18,10 μ ODS, 250×21.2 mm column and a flow rate of 12 mL/min. Pooledfractions were rotary evaporated to remove acetonitrile and lyophilized.Purified samples were subjected to NMR analysis.

[0073]FIG. 2 shows the expected conformations of the relevant protonsand fluorine atoms for2-chloro-9-(2′-deoxy-2′-fluoro-β-D-arabinofuranosyl) adenine(clofarabine) (21) and2-chloro-9-(2′-deoxy-2′-fluoro-α-D-arabinofuranosyl) adenine(epi-clofarabine):

[0074] Based on these conformational assumptions and the Karpusrelationship, the predicted coupling constants of the β-anomer (21) andthe α-anomer (22) should conform to the following relationship:

[0075] a) JH₂F will be large for both the β or α anomers

[0076] b) (JH₁F)_(β)<(JH₁F)₆₀

[0077] c) (JH₁H₂)_(β)>(JH₁H₂)_(α)

[0078] d) JH₂H₃ will be small for both the β or α anomers

[0079] These predictions are borne out by the NMR analysis of thepurified anomers as shown in Table 2, FIG. 3 and FIG. 4. Notably, theexocyclic N₆ protons occur at a predictable chemical shift (7.8-8.0 ppm)for clofarabine (21) and epi-clofarabine (22). Similar N₆ chemicalshifts were reported for other adenine derivatives (Reid et al., Helv.Chim. Acta, 72:1597-1606, 1989). TABLE 2 Relevant Chemical Shifts andCoupling Constants for Anomers† Compound H₂ δ (ppm) H₁ δ (ppm)Clofarabine 5.76 (dt, 1H, J = 63 Hz, 6.31 (dd, J = 15 Hz, J = 5 Hz) J =5 Hz) Epi-Clofarabine 5.61 (dt, J = 57 Hz, 6.19 (dd, J = 19.5 Hz, J = 4Hz) J = 4 Hz)

[0080] The present invention has been shown by both description andexamples. The Examples are only examples and cannot be construed tolimit the scope of the invention. One of ordinary skill in the art willenvision equivalents to the inventive process described by the followingclaims that are within the scope and spirit of the claimed invention.

What is claimed is:
 1. A process for the preparation of a β-nucleosideof the formula:

wherein R¹ is hydrogen, halogen or —OR⁶, wherein R⁶ is a hydroxyprotecting group, R² and R³ are independently hydroxy protecting groups,and R⁵ is a halogen or —NH₂, comprising reacting an α-arabinofuranosylderivative of the formula:

wherein R¹, R² and R³ are as defined above and R⁴ is a leaving groupselected from the group consisting of halogen, alkylsulfonyloxy andarylsulfonyloxy, with an adenine derivative of the formula:

wherein R⁵ is as defined above and the C-6 exocyclic amino group of saidadenine derivative is not protected, in the presence of a solvent and abase, wherein said base has a pKa in water of about 15 or greater, andwherein there is no substantial production of adducts formed by additionof said α-arabinofuranosyl derivative with said C-6 exocyclic aminogroup of said adenine derivative.
 2. The method of claim 1, wherein R⁵is -NH2 and said R⁵ -NH₂ group is unprotected and there is nosubstantial production of adducts formed by addition of saidα-arabinofuranosyl derivative with said C-6 exocyclic amino group ofsaid adenine derivative and/or said unprotected R⁵ —NH₂ group.
 3. Theprocess of claim 1, wherein there is no significant production ofadducts formed by addition of said α-arabinofuranosyl derivative withsaid C-6 exocyclic amino group of said adenine derivative.
 4. The methodof claim 1, wherein R⁵ is —NH₂ and said R⁵ —NH₂ group is unprotected andthere is no significant production of adducts formed by addition of saidα-arabinofuranosyl with said C-6 exocyclic amino group of said adeninederivative and/or said unprotected R⁵ —NH₂ group.
 5. The process ofclaim 1, wherein R⁵ is chloro.
 6. The process of claim 1, wherein R¹ isfluoro.
 7. The process of claim 1, wherein R² and R³ are independentlybenzyl or acetyl.
 8. The process of claim 1, wherein R⁴ is bromo.
 9. Theprocess of claim 1, wherein R⁵ is chloro.
 10. The process of claim 9,wherein R¹ is fluoro.
 11. The process of claim 1, wherein said base ispotassium t-butoxide or potassium t-amylate.
 12. The process of claim 1,wherein said solvent is mixture of two or more solvents from the groupof solvents consisting of t-butyl alcohol, acetonitrile, dichloroethane,dichloromethane, tetrahydrofuran and t-amyl alcohol.
 13. The process ofclaim 1, further comprising de-protecting said β-nucleoside of theformula:

to form a β-nucleoside of the formula:

wherein R¹ and R⁵ are as defined above.
 14. A process for thepreparation of a β-nucleoside of the formula:

wherein R² and R³ are independently hydroxy protecting groups,comprising reacting an α-arabinofuranosyl of the formula:

wherein R² and R³ are as defined above and R⁴ is a leaving groupselected from the group consisting of halogen, alkylsulfonyloxy, andarylsulfonyloxy, with 2-chloroadeinine, wherein the C-6 exocyclic aminogroup of said 2-chloroadenine is not protected, in the presence of baseand a solvent, wherein there is no substantial production of adductsformed by addition of said α-arabinofuranosyl with said C-6 exocyclicamino group of said 2-chloroadenine.
 15. The process of claim 14,wherein R⁴ is bromo.
 16. The process of claim 14, wherein R² and R³ areindependently benzyl or acetyl.
 17. The process of claim 14, whereinsaid base is potassium t-butoxide or potassium t-amylate.
 18. Theprocess of claim 14, wherein said solvent is mixture of two or moresolvents from the group of solvents consisting of t-butyl alcohol,acetonitrile, dichloroethane, dichloromethane, tetrahydrofuran andt-amyl alcohol.
 19. The process of claim 14, wherein said base ispotassium t-butoxide and said solvent is a mixture of t-butyl alcoholand acetonitrile.
 20. The process of claim 14, further comprisingdeblocking the hydroxyl groups of said β-nucleoside of the formula:

to form a β-nucleoside of the formula:


20. A process for the stereoselective preparation of a2′-deoxy-β-nucleoside of the formula:

wherein, R² and R³ are independently hydroxy protecting groups, and R⁵is a halogen or —NH₂, comprising reacting a 2-deoxy-α-arabinofuranosylderivative of the formula:

wherein R⁷ is a halogen and R² and R³ are as defined above, with anadenine derivative salt of the formula:

wherein R⁵ is as defined above and the C-6 exocyclic amino group of saidadenine derivative salt is not protected, in the presence of a solvent,wherein said 2′-deoxy-β-nucleoside is produced in a molar ratio of atleast 10:1 relative to the 2′-deoxy-α-nucleoside anomer represented bythe formula:


21. The process of claim 20, wherein said molar ratio of said2′-deoxy-β-nucleoside to said 2′-deoxy-α-nucleoside is at least 15:1.22. The process of claim 20, wherein said molar ratio of said2′-deoxy-α-nucleoside to said 2′-deoxy-α-nucleoside is at least 20:1.23. The process of claim 22, wherein R⁷ is bromo or chloro.
 24. Theprocess of claim 20, wherein R⁵ is chloro.
 25. The process of claim 20,wherein R² and R³ are independently benzyl or acetyl.
 26. The process ofclaim 20, wherein said adenine derivative salt is formed in situ in saidsolvent by the reaction of a potassium base with an adenine derivativeof the formula:


27. The process of claim 26, wherein said potassium base is potassiumt-butoxide or potassium t-amylate.
 28. The process of claim 20, whereinsaid solvent is selected from the group consisting of t-butyl alcohol, amixture of t-butyl alcohol and acetonitrile, a mixture of t-butylalcohol and dichloroethane, a mixture of dichloroethane andacetonitrile, a mixture of t-amyl alcohol and dichloroethane, a mixtureof t-amyl alcohol and acetonitrile, a mixture of t-amyl alcohol,acetonitrile and dichloromethane and a mixture of t-amyl alcohol,acetonitrile and dichloroethane.
 29. The process of claim 20, whereinthere is no substantial production of adducts formed by addition of said2-deoxy-α-arabinofuranosyl derivative with said C-6 exocyclic aminogroup of said adenine derivative salt.
 30. The process of claim 20,wherein there is no significant production of adducts formed by additionof said 2-deoxy-α-arabinofuranosyl with said C-6 exocyclic amino groupof said adenine derivative salt.
 31. The process of claim 20, furthercomprising purification of said β-nucleoside by recrystallization orpreparation of a slurry in an inert solvent.
 32. The process of claim31, wherein said purification of said β-nucleoside comprises reslurryfrom methanol or recrystallization from mixture of butyl acetate andheptane.
 33. The process of claim 20, further comprising de-protectingsaid 2′-deoxy-β-nucleoside of the formula:

to form a 2′-deoxy-β-nucleoside of the formula:

wherein R⁵ is as defined above.
 34. A process for the stereoselectivepreparation of a 2′-deoxy-β-nucleoside of the formula:

wherein, R² and R³ are independently hydroxy protecting groups,comprising reacting a 2-deoxy-α-arabinofuranosyl derivative of theformula:

wherein R² and R³ are as defined above, with an adenine derivative saltof the formula:

wherein the C-6 exocyclic amino group of said adenine derivative salt isnot protected, in the presence of a solvent, wherein said2′-deoxy-β-nucleoside is produced in a molar ratio of at least 10:1relative to the 2′-deoxy-α-nucleoside anomer represented by the formula:


35. The process of claim 34, wherein said molar ratio of said2′-deoxy-β-nucleoside to said 2′-deoxy-α-nucleoside is at least 15:1.36. The process of claim 34, wherein said molar ratio of said2′-deoxy-β-nucleoside to said 2′-deoxy-α-nucleoside is at least 20:1.37. The process of claim 34, wherein R² and R³ are independently benzylor acetyl.
 38. The process of claim 34, wherein said adenine derivativesalt is formed in situ in said solvent by the reaction of a potassiumbase with an adenine derivative of the formula:


39. The process of claim 38, wherein said potassium base is potassiumt-butoxide or potassium t-amylate.
 40. The process of claim 34, whereinsaid solvent is selected from the group consisting of t-butyl alcohol, amixture of t-butyl alcohol and acetonitrile, a mixture of t-butylalcohol and dichloroethane, a mixture of dichloroethane andacetonitrile, a mixture of t-amyl alcohol and dichloroethane, a mixtureof t-amyl alcohol and acetonitrile, a mixture of t-amyl alcohol,acetonitrile and dichloromethane and a mixture of t-amyl alcohol,acetonitrile and dichloroethane.
 41. The process of claim 34, whereinthere is no substantial production of adducts formed by addition of said2-deoxy-α-arabinofuranosyl derivative with said C-6 exocyclic aminogroup of said adenine derivative salt.
 42. The process of claim 34,wherein there is no significant production of adducts formed by additionof said 2-deoxy-α-arabinofuranosyl with said C-6 exocyclic amino groupof said adenine derivative salt.
 43. The process of claim 34, furthercomprising purification of said β-nucleoside by recrystallization orpreparation of a slurry in an inert solvent.
 44. The process of claim43, wherein said purification of said β-nucleoside comprises reslurryfrom methanol or recrystallization from mixture of butyl acetate andheptane.
 45. The process of claim 43, further comprising de-protectingsaid 2′-deoxy-β-nucleoside of the formula:

to form a 2′-deoxy-β-nucleoside of the formula:


46. A process for the preparation of a composition comprising2-chloro-9-(2′-deoxy-2′-fluoro-β-D-arabinofuranosyl) adenine comprising:(1) reacting 3,5-O-dibenzoyl-2-deoxy-2-fluoro-α-D-arabinofuranosylbromide with a 2-chloroadeinine potassium salt of the formula:

wherein the exocyclic amino group of said 2-chloroadenine potassium saltis not protected, in the presence of a solvent to form2-chloro-9-(3′,5′-O-dibenzoyl-2′-deoxy-2′-fluoro-β-D-arabinofuranosyl)adenine; (2) deprotecting said2-chloro-9-(3′,5′-O-dibenzoyl-2′-deoxy-2′-fluoro-β-D-arabinofuranosyl)adenineto from 2-chloro-9-(2′-deoxy-2′-fluoro-β-D-arabinofuranosyl) adenine;and (3) isolating said2-chloro-9-(2′-deoxy-2′-fluoro-β-D-arabinofuranosyl) adenine to form acomposition comprising2-chloro-9-(2′-deoxy-2′-fluoro-β-D-arabinofuranosyl) adenine.
 47. Theprocess of claim 46, wherein said composition comprising2-chloro-9-(2′-deoxy-2′-fluoro-β-D-arabinofuranosyl) adenine comprisessubstantially pure 2-chloro-9-(2′-deoxy-2′-fluoro-β-D-arabinofuranosyl)adenine wherein the ratio of said2-chloro-9-(2′-deoxy-2′-fluoro-β-D-arabinofuranosyl) adenine to said2-chloro-9-(2′-deoxy-2′-fluoro-α-D-arabinofuranosyl) adenine as measuredby high pressure liquid chromatography and spectrophotometric analysisis at least 99:1.
 48. The process of claim 46, further comprisingisolating said2-chloro-9-(3′,5′-O-dibenzoyl-2′-deoxy-2′-fluoro-β-D-arabinofuranosyl)adenine before deprotecting said2-chloro-9-(3′,5′-O-dibenzoyl-2′-deoxy-2′-fluoro-β-D-arabinofuranosyl)adenine.
 49. The process of claim 48, wherein said isolating said2-chloro-9-(3′,5′-O-dibenzoyl-2′-deoxy-2′-fluoro-β-D-arabinofuranosyl)adenine comprises a re-slurry procedure.
 50. The process of claim 46,wherein said isolating said2-chloro-9-(2′-deoxy-2′-fluoro-β-D-arabinofuranosyl) adenine comprisesrecrystallising said2-chloro-9-(2′-deoxy-2′-fluoro-β-D-arabinofuranosyl) adenine.
 51. Theprocess of claim 46, wherein said 2-chloroadeinine potassium salt isformed in situ in said solvent by the reaction of a potassium base with2-chloroadenine.